Defrost sensor and control circuit

ABSTRACT

An apparatus for regulating frost buildup in a refrigeration apparatus. A photocell and lamp are mounted to function within an open housing having essentially non-reflective interior wall surfaces. The housing is located adjacent the refrigeration apparatus permitting cold air to circulate therethrough and allow frost buildup on such surfaces. As front forms thereon, illuminance intensity of the lamp light within the housing increases due to the light scattering characteristics of the frost causing the photocell to function to initiate the defrost cycle.

United States Patent [191 Hansen et al. July 9, 1974 [5 DEFROST SENSOR AND CONTROL 3,280,577 10/1966 Kobayashid 62/151 CIRCUIT P Ex M P r A I rzmary ammer-- eyer er in [75] Inventors James EZHansen Oak Creek Attorney, Agent, or Firm-H. R. Rather; Wm. A.

P. La Pointe, Mequon, both of WIS. Autio V [73] Assignee; Cutter-Hammer, Inc., Milwaukee, [57 ABSTRACT [22] Filed: Sept. 29, 1972 An apparatus for regulating frost buildup in a refrigeration apparatus. A photocell and lamp are mounted to [21] Appl' 293452 function within an open housing having essentially non-reflective interior wall surfaces. The housing is [52] U.S. Cl 62/140, 62/ 151, 62/275 located adjacent the refrigeration apparatus permit- [51] Int. Cl. F25d 21/02 ting cold air to circulate therethrough and allow frost [58] Field of Search 62/ 140, 154, 151, 227, buildup on such surfaces. As front forms thereon, illu- I 62/ 229, 275, 276 minance intensity of the lamp light within the housing increases due to the light scattering characteristics of [56] References Cited the frost causing the photocell to function to initiate UNITED STATES PATENTS the defrost y 3,1ss,s2s 6/1965 Wayne.... 62/140 9 Cl aii ns 3 l) lfa wing igr S BACKGROUND OF THE INVENTION 1. Field of the Invention This invention generally relates to defrosting controls for refrigeration apparatus and more particularly to an improvement in the apparatus for sensing and controlling frost buildup therein.

2. Description of the Prior Art Refrigeration systems typically require periodic removal of frost from cooling elements which are used to absorb heat from the refrigerated zone since frost accumulation thereon decreases the cooling efiiciency of the system. Various methods have been devised to control removal of such frost. For example, timers have been employed to initiate defrost cycle after passage of preselected time intervals. It should be noted that such devices lead to excessive cycling-in dry weather when frost buildup is infrequent andto overloading in damp weather when frequency defrosting is required, and consequently, these timers have not been entirely satisfactory from a frost removal standpoint. More recently, various solid state and hybrid control circuits having frost sensing elements within the refrigerated area for sensing frost buildup have been adapted to coordinate cooling and defrosting functions in response to signals from such sensors. However, those proposed generally involve complicated sensing arrangements and circuitry, and although they serve the purpose for which they are intended, they are generally expensive to manufacture andmaintain.

SUMMARY OF THE INVENTION The general object of the present invention is to provide an improved frost control for regulating a defrost cycle in a refrigeration apparatus which circumvents the problems heretofore noted.

More specifically, it is an object to provide a frost control wherein a frost sensor is responsive to illuminance intensity generated by frost accumulation for energizing a defrost device to melt such frost.

In achieving these and other objects as will become apparent hereinafter, one embodiment of the present housing walls increases and is sensed by the photocell which is positioned to receive indirect light which is emitted from the lamp within the housing and reflected and scattered off the white frost accumulation.

BRIEF DESCRIPTION OF THE DRAWINGS In describing the present invention, reference will be made to the accompanying drawings forming a part of the instant disclosure wherein:

FIG. l-is a perspective view of a typical refrigeration apparatuspartially broken away to illustrate the: frost sensor and control of the present invention in combination with such apparatus.

FIG. 2 is a perspective view partially broken away of the frost sensor device constructed in accordance with the present invention.

FIG. 3 is a schematic illustration of the refrigeration and defrost control circuit diagram embodying the present invention.

DETAILED DESCRIPTION OF INVENTION frost sensor device 10 and control 14, it will be helpful atthe outset to describe generally the overall cooling operation of a refrigeration apparatus in which the new frost sensor device 10 is especially advantageous. Such an apparatus normally includes the aforementioned cooling element 12 comprising coils 16 through which a heat exchanging fluid or refrigerant flows and fins 18 mechanically attached to the coils l6 toenhance the heat exchange function. The coils 16 and fins 18 are positioned within the area to be refrigerated to absorb heat from air passing thereby. The heat exchange fluid removes such heat to a heat exchanging device 19 includinga compressor motor CM in a manner commonly known and practiced throughout the industry. Efficiency of such a system drops considerably as frost accumulation and buildup occurs on the cooling element 12. Consequently, it is important to remove such frost to restore cooling power. I

Having briefly described the general overall cooling operation of a refrigeration apparatus, it will be helpful to refer now to the constructional features of the frost sensor device 10 and control 14 of the present invention illustrated in detail in FIGS. 2 and 3 of the drawings. The frost sensor device 10 comprises a housing 20, a light source or lamp 22 and a photosensitive de vice such as a photoconductive cell 24 operatively affixed thereto. The .control 14 is operatively connected to the device 10 and in conjunction therewith regulates the cooling and defrosting stages of the refrigeration apparatus 13 to maximize its cooling efficiency.

The housing 20 is constructed of a U -shaped, open ended member 26 secured to a mounting plate 28 in box-like fashion as shown in FIG. 2. The housing ends are open to permit air convection therethrough so that frost builds up therein concurrently with that accumulating on the coils l6 and fins 18 of the cooling element 12. The housing 20, with the exception of base portion 29, is constructed of a material having good heat conducting qualities such as aluminum so that it can instantaneously simulate the conditions of the cooling element 12 for accumulating frost. The base portion 29 upon which the lamp 2. and cell 24 are mounted is of low thermal conductivity material to aid in retention of non-reflective coating such as black matte paint to re-' duce illuminance characteristics within the housing 20 prior to frost buildup. The finish 30 is automatically cleared of frost and other foreign particles from the washing action of melted frost run-off and consequently is self-cleaning and maintenance-free.

The lamp 22 is mounted as shown in FIG. 2 to base portion 29 to provide illumination continuously within the housing and is of the type that illuminates in all directions asopposed to focusing type lamps. A lamp of very low power requirement is selected to ensure long lamp life. It should be noted that lamp operation need not be continuous in which case a timer or other switching device (not shown) would be utilized to briefly energize the lamp at preselected time intervals thereby increasing lamp life. Under extreme environmental conditions, intermittent lamp operation may permit frost buildup thereon hampering its operation, and in such cases, a heater or small resistor R in FIG. 3 is connected in parallel with lamp 22 as shown in FIG. 3 to provide added heat. R is selected to perform this function without hindering frost accumulation on the housing wall surfaces 32.

The photocell 24 is designed to integrate light of low value within the housing 20 and is mounted generally adjacent and ahead of lamp 22 in a position to measure scattered and diffused light as shown in FIG. 2. The electrical resistance of the cell 24 decreases appreciably from exposure to increasing light radiation or'illuminance within the housing 20. Consequently, cell 24 is selected to trigger the defrost cycle whenever such illuminance reduces its resistance to a reference level. Positioning of the cell 24 near the lamp 22 as shown in FIG. 2 generally assures no frost will build up on the cell 24 to interfere with its intended operation. Under extreme environmental conditions, R may be utilized to provide added head to maintain both the lamp 22 and cell 24 clear from frost.

. Consequently, it can be seen that, as frost builds up within the housing 20, such accumulation sharply increases scattering of light therein to an illuminance level to which the photoconductive cell 24 responds by turning on the defrost cycle described hereinafter with reference to FIG. 3.

Now referring to FIG. 3, the compressor motor CM is connected across A.C. supply lines LI and L2 in series with main terminal Al and Kl of a semiconductor device such as a bidirectional thyristor Q1 of the type sold under the trade name Triac. Defrost heater H is similarly connected across lines L1 and L2 in series with the main terminal A2 and K2 of a second thyristor Q2 like 01. The control gate G1 of O1 is connected to line L1 in series with an electric thermostat TH, a resistor R1 and normally closed contacts RC1 of a relay RC. Control gate G2 of O2 is connected to line L1 through resist or R2 and normally open contacts RC2 of relay RC.

When lines L1 and L2 are energized and the operating coil RC3 of relay RC is deenergized, it may be assumed that Q1 will be conducting if the electrical contacts of thermostat TH are closed. When Q1 is con ducting, full wave A.C. current will be supplied to compressor motor CM. Also when relay RC is deenergized, Q2 will be held non-conducting to interrupt the flow of AC. current to defrost heater H.

The remainder of the control depicted in FIG. 3, and now to be described, provides for the appropriate energization of relay RC to stop the refrigeration cycle and initiate defrosting of the cooling element 12, and provides deenergization of reiay RC upon completion of such defrosting action.

The control for relay RC requires a source of rectified, filtered and regulated voltage which is provided by a power transformer T having a primary winding P and a center tapped secondary winding S, half wave rectifier diodes Dll and D2, a capacitor C1, a resistor R3, and a zener diode D3. Primary winding P of the transformer is connected across A.C. lines L1 and L2, and the opposite ends of secondary winding S are respectively connected through diodes Di and D2 and resistor R3 to a positive potential conductor 33. Capacitor Cl is connected at its high potential plate to the point common between diodes D1 and D2 and resistor R3, and at its low potential terminal to center tap CT of winding S and to ground. Zener diode D3 has its anode connected to center tap CT and ground and its cathode to the point common between resistor R3 and conductor 33.

Conductor 33 is connected through a current limiting resistor R7 to a conductor 34 which provides a somewhat lower positive D.C. potential for certain of the relay control circuitry. Lamp 22 has one terminal connected through a conductor 35 and has its other terminal connected through a conductor 36 to ground. Photoconductive cell 24 is connected at one of its terminals through a conductor 38 to the gate G3 of a semiconductor device such as a programmable unijunction transistor Q3 of the type sold under the trade name of PUT. The cathode K3 of Q3 is connected to ground through conductor 36, and its anode gate A3 is connected to the point common between voltage divider resistor R4 and R5 which are connected in series across the conductors 34 and 36 and to the anode of a zener diode D4.

Control coil RC3 of relay RC is connected at one end to conductor 34 and at its other end to the cathode of Y Zener diode D4. The zener break-down voltage of D4 is selected to be slightly higher than that across resistor R4 to prevent undesired current flow through coil RC3. A variable resistor R6 has its resistance element connected at one end to conductor 34 and at its other end to conductor 38. It will be seen that resistor R6 and cell 24 are thus connected in series across conductors 34 and 36 and provide a voltage divider for the gate G3 of PUT Q3.

The common point between voltage divider resistors R4 and R5 provides a fixed reference voltage Va to which the anode A3 of PUT Q3 and the anode of zener diode D4 are subjected. With no frost buildup on the cooling element 112, it may be assumed that the resistance of cell 24 due to the low intensity of illuminance will be sufficiently high to subject gate G3 to a resultant voltage Vg which is higher than the reference voltage Va impressed on anode A3. Consequently, PUT Q3 will be held nonconducting.

A capacitor C2 is connected between conductors 34 and conductor 38 and gate G3 to suppress transient voltages that might otherwise cause false triggering of PUT Q3. A diode D5 is connected across relay coil RC3 and zener diode D4 to clamp induced voltages when coil RC3 is suddenly deenergized at the end of a defrost cycle.

Now let it be assumed that during operation of compressor motor CM, frost builds up oncooling element 12. This results in increased illuminance in the housing 20 as aforedescribed and consequently photoconductive cell 24 decreases in resistance thereby decreasing the magnitude of voltage Vg to which gate G3 of PUT Q3 is subjected. When voltage Vg decreases to the value of voltage Va, PUT O3 is turned on and is latched in on, a fully conducting state in a manner similar to a silicon controlled rectifier. Conduction of PUT Q3 brings the potential of its anode A3 close to ground, and the potential drop across zener diode D4 exceeds its breakdown voltage to cause current flow through relay coil RC3. Opening of contacts RC1 and closure of contacts RC2 then results. Consequently, Q1 is rendered non-conducting to deenergize motor CM, if then energized, and O2 is rendered conducting to en ergize defrost heater H. Accordingly, any refrigeration cycle then existing is stopped and the defrost cycle initiated.

As the defrost cycle proceeds, frost on the cooling element 12 will be melted, thereby decreasing the illuminance in housing 20. This results in an increase in resistance of photoconductive cell -24, but even though it rises to a value causing voltage Vg to rise to its initial value exceeding that before obtaining at the junction of resistors R4 and R5 and R4, it cannot render PUT Q3 conducting again until the flow of current through the anode A2 of O3 is reduced below its valley current value.

The remaining portion of the control circuitry now to be described provides for stopping of the control flow through relay coil RC3 and PUT Q3 when the defrost cycle has effected removal of a sufficient amount of frost from cooling element 12. i

A semiconductor device such as an N-P-N transistor Q4 has its collector connected to conductor 34 and its emitter to ground. The base of O4 is connected in series with a zener diode D6 to the point common between the lower end of variable.resistor R8 and the upper terminal of a positive temperature coefficient thermistor PTC which is mounted on cooling element 12. Resistor R8 and thermistor PTC act as a voltage divider and subject the cathode of zener diode D6 to-a voltage which increases with the resistance of thermistor FTC.

Now let it be assumed as the defrost cycle proceeds, that the frost has melted and the cooling element continues to rise in temperature due to the heat generated by the heater H. When the temperature of thermistor PT C reaches its curie point, approximately 65F, a very sharp rise in its resistance occurs causing a correspond ing sharp rise in voltage at the cathode of zener diode D6. When this latter voltage exceeds the breakdown voltage across zener diode D6, current will then flow from conductor 33 through resistor R8, and diode D6 into the base of Q4 to render the latter substantially fully conducting.

When transistor O4 is conducting, current flowing through resistor R7 is shunted through the collectoremitter circuit of O4 to ground, thereby shutting off current flow through relay coil RC3 and PUT Q3 and resistors R4 and R5. Consequently, coil RC3 is deener gized and contacts RC1 reclose and contacts RC2 reopen. Accordingly, defrost heater H is deenergized to stop the defrost cycle and the gate circuit of Triac O1 is again readied to effect conduction of the latter whenever the thermostat TH functions to complete the gate circuit.

It will be observed that with flow of current into the tively reset to a nonconducting state and will again be effective to initiate the defrost cycle in the manner hereinbefore described.

lfdesired, Triacs Q1 and 02 can be replaced by electromagnetic relays or contactors, and the contacts RC1 and RC2 of relay RC would then be appropriately connected in the energizing circuits for the operating coils of such relays or contactors in a well-known manner.

In summary, it should be appreciated by one skilled in the art that the present invention provides a simple, inexpensive and highly responsive frost sensor device and control therefor for sensing frost and initiating a defrost cycle only when needed within a refrigeration apparatus.

It will be apparent that various details of the illustrated forms of the present invention may be varied without departing from the inventive concept. It will accordingly be understood that it is intended to embrace within the scope of this invention such modifications as may be embraced by the skill of the art.

We claim: a

1. In a frost sensor for refrigeration apparatus having a coolingelement therefor which refrigerant is circulated, and which is susceptible to frost build-up thereon, and a'defrost device adapted when energized anode A3 of PUT Q3 stopped, the latter is then etfecto remove frost from said cooling element, the combination comprising:

a housing having irregular inner wall surfaces of low reflective ability on which frost can accumulate and being adapted to be mounted adjacent said cooling element in frost forming relationship therewith.

a light source operatively mounted to said housing to illuminate said inner wall surfaces of said housing;

fused within said housing for sensing by said photosensitive device.

3, A frost sensor according to claim 1 wherein said photosensitive device is positioned adjacent and ahead of said light source to prevent light from being directly transmitted from said light source to said photosensitive device.

4. The combination according to claim 1 together with a heating means operatively connected to said housing for providing heat to said photosensitive device and said light source to maintain said device and source free from frost accumulation. r

5. In a refrigeration apparatus, the combination with a cooling element through which refrigerant is circulated and upon whichfrost issusceptible of build-up, means energizablejto circulate refrigerant through said cooling element and energizable to cause melting of frost on the latter, of meansinciuding control means for energizing and deenergizing the first mentioned means and said defrost means, said control means having a frost sensor comprising:

a. a partially open housing mounted adjacent said cooling element and having inner walls in frost receiving relation thereto;

b. a light source mounted in said housing and illuminating said inner wall surfaces thereof;

0. a photosensitive cell mounted in said housing and arranged to be responsive to change in the level of illuminance in said housing as frost forms on said inner wall surfaces;

d. first semiconductive means responsive to said photosensitive cell to cause said defrost means to be energized and said first mentioned means to be deenergized when illuminance reaches a predetermined level within said housing;

e. second semiconductor means responsive to decrease of illuminance in said housing resulting from melting of frost to render said first semiconductor means inactive and thereby effect deenergization of said defrost means and activation of said first mentioned means to resume the refrigeration proergizable and deenergizable to provide response of said third and fourth semiconductor means alternately, wherein the energizing winding of said relay is in circuit with the anode-cathode circuit of said unijunctiontransistor, and wherein the gate terminal of the latter is in circuit with said photoconductive cell and is responsive to the resistance of the latter decreasing to a predetermined level to cause conduction of said unijunction transistor and energization of the relay energizing winding.

7. A refrigeration apparatus according to claim 6 wherein said second semiconductor means is a transistor having its emitter-collector terminals connected in circuit with said unijunction transistor and said relay and wherein said control means further includes a temperature sensitive device contiguous with said cooling element and in circuit with said unijunction transistor and second semiconductor transistor to shunt essentially all current flow away from said unijunction transistor to ground at the end of a defrost cycle thereby deenergizing said unijunction transistor and said relay to cause said third semiconductor means to be deenergized to end defrost cycle and said fourth semiconductor means to be energized to resume the refrigeration process as demanded by the system.

8. A refrigeration apparatus according to claim 7, wherein said third and fourth semiconductor devices are bidirectional thyristors and wherein said temperature sensitive device is a positive temperature coefficient thermistor.

9. The combination according to claim 5, together with a heating means operatively connected to said housing for providing heat to said photosensitive cell and said light source to maintain said cell and source free from frost accumulation.

Q53 3?" i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3, 822, 560 Dated December 10 1974 Inventm-(sy James E. Hansen and Joe]. P. La Pointe It is certified that error appears in the above-identified, patent and that said Letters Patent are hereby corrected as shown below:

In the abstract line 7 reads:

"A's front forms thereon," and should read:

-- As frost forms thereon,

In the specification Column 1, line 21' reads: 7 r "weather when frequency defrosting? and should read:

weather when frequent defrosting C Column 3, line 35 reads:

"to provide added head" and should read:

-- to provide added heat Column 3, line 55 reads:

' "resist or R2" and should read:

,--- resistor R2 In the claims Claim line 52 reads:

"A frost sensor according to claim l" n and should read:

A 'frost sensor according to "claim 2 Signed and sealed this 4th day of February 1975.

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. In a frost sensor for refrigeration apparatus having a cooling element therefor which refrigerant is circulated, and which is susceptible to frost build-up thereon, and a defrost device adapted when energized to remove frost from said cooling element, the combination comprising: a housing having irregular inner wall surfaces of low reflective ability on which frost can accumulate and being adapted to be mounted adjacent said cooling element in frost forming relationship therewith. a light source operatively mounted to said housing to illuminate said inner wall surfaces of said housing; a photosensitive device operatively mounted to said housing to sense the level of illuminance therein when frost builds up on said inner wall surfaces and responsive to initiate removal of frost from said cooling element when illuminance reaches a preselected level within said housing, and said photosensitive device being directed away from said light source to prevent direct impingement of light thereon.
 2. A frost sensor according to claim 1 wherein said light source emits scattered light therefrom to be diffused within said housing for sensing by said photosensitive device.
 3. A frost sensor according to claim 1 wherein said photosensitive device is positioned adjacent and ahead of said light source to prevent light from being directly transmitted from said light source to said photosensitive device.
 4. The combination according to claim 1 together with a heating means operatively connected to said housing for providing heat to said photosensitive device and said light source to maintain said device and source free from frost accumulation.
 5. In a refrigeration apparatus, the combination with a cooling element through which refrigerant is circulated and upon which frost is susceptible of build-up, means energizable to circulate refrigerant through said cooling element and energizable to cause melting of frost on the latter, of means including control means for energizing and deenergizing the first mentioned means and said defrost means, said control means having a frost sensor comprising: a. a partially open housing mounted adjacent said cooling element and having inner walls in frost receiving relation thereto; b. a light source mounted in said housing and illuminating said inner wall surfaces thereof; c. a photosensitive cell mounted in said housing and arranged to be responsive to change in the level of illuminance in said housing as frost forms on said inner wall surfaces; d. first semiconductive means responsive to said photosensitive cell to cause said defrost means to be energized and said first mentioned means to be deenergized when illuminance reaches a predetermined level within said housing; e. second semiconductor means responsive to decrease of illuminance in said housing resulting from melting of frost to render said first semiconductor means inactive and thereby effect deenergization of said defrost means and activation of said first mentioned means to resume the refrigeration process as demanded, and f. third and fourth semiconductor means in circuit respectively with first mentioned means, said defrost means, and said first semiconductor means, said third and fourth semiconductor means being responsive to actuation and deactuation and said first semiconductor means to effect corresponding energization and deenergization of said defrost means and said first mentioned means, selectively.
 6. A refrigeration apparatus according to claim 5, wherein said first semiconductor means comprises a programmable unijunction transistor having anode, gate and cathode terminals, wherein said control means further includes an electromagnetic relay which is energizable and deenergizable to provide response of said third and fourth semiconductor means alternately, wherein the energizing winding of said relay is in circuit with the anode-cathode circuit of said unijunction transistor, and wherein the gate terminal of the latter is in circuit with said photoconductive cell and is responsive to the resistance of the latter decreasing to a predetermined level to cause conduction of said unijunction transistor and energization of the relay energizing winding.
 7. A refrigeration apparatus according to claim 6 wherein said second semiconductor means is a transistor having its emitter-collector terminals connected in circuit with said unijunction transistor and said relay and wherein said control means further includes a temperature sensitive device contiguous with said cooling element and in circuit with said unijunction transistor and second semiconductor transistor to shunt essentially all current flow away from said unijunction transistor to ground at the end of a Defrost cycle thereby deenergizing said unijunction transistor and said relay to cause said third semiconductor means to be deenergized to end defrost cycle and said fourth semiconductor means to be energized to resume the refrigeration process as demanded by the system.
 8. A refrigeration apparatus according to claim 7, wherein said third and fourth semiconductor devices are bidirectional thyristors and wherein said temperature sensitive device is a positive temperature coefficient thermistor.
 9. The combination according to claim 5, together with a heating means operatively connected to said housing for providing heat to said photosensitive cell and said light source to maintain said cell and source free from frost accumulation. 